In the commercial production of polyethers by the alkoxylation of suitable starter compounds containing active hydrogen atoms, the resulting crude product contains from about 0.1 to 2%, by weight, of residual water, from about 0 to 2% of solvent (generally an aromatic solvent, such as xylene or toluene) and odorous substances (largely propionaldehyde, propionic acid, dioxolanes and allyl ethers). This crude product is obtained after neutralization of the reaction mixture, preliminary dehydration and subsequent filtration of inorganic salts. Although the odorous substances are generally present in quantities of only up to 1%, by weight, they nevertheless impart intensively aromatic odor to the polyethers. In conventional processes, the odorous substances may largely be removed by treating the crude polyether in a stripping column. The crude polyether may then be freed from residual water and residual solvent in a thin film evaporator. For economic reasons, the crude polyether is generally subjected to only a single working-up stage in the thin film evaporator (i.e. without pretreatment in a stripping column). Unfortunately, it is extremely difficult to remove all the solvent residues in this way and the odorous substances are only removed to a minimal extent. Another disadvantage of the thin film evaporator is its considerable cost and outlay and also the difficulty of sealing off the rotating parts. In addition, thin film evaporators have to be operated at very low pressure (generally from 1 to 10 mbars), which necessitates an elaborate vacuum-generating system and complicates condensation of the volatile fractions. If, however, the thin film evaporators are operated under pressures above 10 mbars, the operating temperature has to be selected so high that even minor leaks in the system result in damage to the polyether by oxidation.
In principle, falling film evaporators may also be used for working-up crude polyethers. Unfortunately, they are attended by substantially the same disadvantages as thin film evaporators for the application in question. In addition, problems are involved in uniformly distributing the crude polyether over the evaporation surface.